Non-contacting seals are successfully employed in gas compressors to provide a seal against loss of process gas. Such a seal is shown, for example, in U.S. Pat. No. 4,212,475 to Sedy.
In practice, non-contacting seals are often arranged in an assembly having two spaced apart sets of relatively rotating rings which, in some applications, define an intermediate chamber containing a pressurized barrier gas. The seal ring sets each include a mating ring and an axially movable primary ring. Grooves formed on the face of one of the rings of each set communicate with the barrier gas. One seal set pumps gas from the buffer chamber toward the process fluid. The other pumps toward the atmosphere. An example of such a seal is the double Type 28 gas compressor seal manufactured by John Crane Inc., Morton Grove, Ill.
Gas compressor seals of the type described are configured such that on loss of buffer or barrier gas, the inboard seal opens and defines a leakage path to the intermediate chamber. The outboard pair of seal rings operates as a non-contacting seal and pumps a controlled amount of the process gas between the faces. However, since loss of buffer gas often results from failure of the outboard seal, opening of the faces of the inboard seal could cause undesirable leakage through the buffer chamber to atmosphere.
Non-contacting seals that operate on a film of gas have more recently been employed to seal liquid in pump applications. An example is found in U.S. Pat. No. 5,375,853. There, spaced seal sets define a buffer chamber for gas at a pressure higher than the process. The inboard seal set pumps the gaseous barrier across the relatively rotatable faces toward the process fluid. The outboard set pumps the barrier gas toward the atmosphere. John Crane Inc. manufactures and sells such a seal arrangement for pumps under the designation T-2800.
In pump applications, the inboard seal set is configured such that on loss of buffer pressure the inboard seal closes and operates as a contacting seal sufficiently long to permit shut-down of the pump. Such an arrangement would not be feasible in the gas compressor environment because the resulting face contact could affect structural integrity.
It has been determined, however, that in gas compressor and similar applications, the process fluid can effectively be contained upon a pressure reversal if the inboard seal ring set were arranged to continue to operate as a non-contacting seal with the process fluid providing the requisite lift. In this way only a small, controlled quantity of process gas would pass to the buffer chamber, thereby, minimizing loss to atmosphere. The present invention is directed to a seal assembly arranged to provide this capability.
The present invention provides a non-contacting seal arrangement between a housing and relatively rotatable shaft to contain a process fluid in the housing which, on loss of barrier fluid pressure, the inboard seal continues to operate as a non-contacting seal. The seal arrangement includes a pair of spaced sets of relatively rotating rings defining an intermediate chamber to receive a barrier gas at a pressure exceeding process fluid pressure. Each set includes a non-rotatable ring and a rotatable ring, one of the rings being movable axially relative to the other. Each ring of each set defines a generally radial annular sealing face in relatively rotating sealing relation to the sealing face of the other ring of the set at a sealing interface. One of the rings of at least one set has a pumping mechanism thereon arranged to pump barrier gas from the intermediate chamber between the interface. That set is adapted to be disposed to pump barrier gas toward the process fluid in the housing. The pumping mechanism of the ring is further configured to pump process fluid between the interface toward the intermediate chamber when the process fluid pressure exceeds the pressure of the barrier gas.
More particularly, the invention may include a retainer to support the axially movable ring of the set disposed to pump barrier gas toward the process fluid. The retainer and ring define an axially elongated annular pocket. An O-ring seal is disposed in the pocket and provides a secondary seal between the retainer and the ring. It is sized such that it has a cross-sectional diameter that is smaller than both the axial and radial extent of the pocket.
The axially movable ring of the seal set disposed to pump barrier fluid toward the process may include a first portion defining the radially directed sealing face, a second portion supporting the ring for axial movement, and an intermediate portion configured to decouple said first and second portions to ensure a parallel relationship between the relatively rotating sealing faces under varying conditions of operating pressure and temperature.
The invention further contemplates the method of sealing using the seal assembly comprising providing a barrier gas in the intermediate chamber at a pressure in excess of the pressure of said process fluid, pumping barrier gas from the intermediate chamber between the interface toward the process fluid when the pressure of the barrier gas exceeds the pressure of the process fluid, and pumping process fluid between the interface toward the intermediate chamber when the process fluid pressure exceeds the pressure of the barrier gas.